Semiconductor fabrication processes are typically conducted with the substrate supported within a chamber under controlled conditions. For many processes, semiconductor substrates (e.g., silicon wafers) are heated inside the process chamber. For example, substrates can be heated by direct physical contact with a heated wafer holder and/or by radiation from a radiant heating source. “Susceptors,” for example, are wafer supports that absorb radiant heat and transmit absorbed heat to the substrate.
In a typical process, a reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of reactant material on the wafer. Through sequential processing, multiple layers are made into integrated circuits. Other exemplary processes include sputter deposition, photolithography, dry etching, plasma processing, and high temperature annealing. Many of these processes require high temperatures and can be performed in similar reaction chambers.
Various process parameters must be controlled carefully to ensure high quality in deposited films. One critical parameter is the temperature of the wafer during the processing. During CVD, for example, there is a characteristic temperature range in which the process gases react most efficiently for depositing a thin film onto the wafer. Temperature control is especially critical at temperatures below the mass transport regime, such as silicon CVD between about 500 C and 900 C (kinetic regime, about 500 C to 900 C for silicon CVD using silane). In this kinetic regime, if the temperature is not uniform across the surface of the wafer, the deposited film thickness will be uneven.
In recent years, single-wafer processing of large diameter wafers has become more widely used for a variety of reasons, including the need for greater precision in process control than can be achieved with batch-processing. Typical wafers are made of silicon, most commonly with a diameter of about 150-mm (6 inches) or of about 200-mm (8 inches) and with a thickness of about 0.725 mm. Recently, larger silicon wafers with a diameter of about 300 mm (12 inches) and a thickness of about 0.775 mm have been introduced, as they exploit, even more efficiently, the benefits of single-wafer processing. Even larger wafers are expected in the future.
The inventor has found many quality control problems affecting yield result from handling issues associated with susceptors, including substrate slide, stick and curl. These occur during placement and subsequent removal of substrates in high temperature process chambers.
Slide occurs during drop off when a cushion of gas in the susceptor recess or pocket is unable to escape fast enough to allow the substrate to fall immediately onto the susceptor. The substrate floats momentarily above the susceptor as the gas slowly escapes, and it tends to slide off center. Thus, the substrate may not rest in the center of the pocket where it was intended, and uneven heating of the substrate may result. Sliding to the edge of a susceptor pocket causes local cooling where the substrate is in contact with the pocket edge and results in poor thickness uniformity, poor resistivity uniformity and crystallographic slip, depending on the nature of the layer being deposited. These non-uniformities, due to inconsistencies in the wafer drop position, greatly increase the difficulty in optimal tuning of the process. Similarly, non-uniformities in temperature can cause non-uniformities in etch, anneal, doping, oxidation, nitridation and other fabrication processes.
Conversely, during pickup, stick occurs when the substrate clings to the underlying support because gas is slow to flow into the small space between the wafer and the surface of the pocket. This creates a vacuum effect between the substrate and the support as the substrate is lifted. Stick is a potential contributor to particle contamination and, in extreme cases, has caused lifting of the substrate holder on the order of 1 to 2 mm.
Curl is warping of the substrate caused by a combination of both radial and axial temperature gradients in the substrate. Severe curl can cause the substrate to contact the bottom side of a Bernoulli wand, and can similarly affect interaction with other robot end effectors. In the case of a Bernoulli wand, the top side of the substrate can scratch the Bernoulli wand and cause particulate contamination, significantly reducing yield. The design and function of a Bernoulli wand are described in U.S. Pat. No. 5,997,588 and are included by reference herein.
FIGS. 1A and 1B show a wafer 1 supported upon a susceptor 100, wherein the susceptor 100 has a gridded support surface G. Referring initially to FIG. 1A, a portion of the wafer 1, close to a peripheral edge 2 thereof, is shown on the grid G. An upper surface of the grid G is defined by a plurality of projections 3 separated from one another in two dimensions by a plurality of grid grooves. These projections 3 are recessed with respect to the upper surface of an annular shoulder 4 surrounding the grid. For a 200-mm wafer, the depth of this recess or pocket is about 0.018 inches (0.457 mm), while the thickness of a 200-mm wafer is about 0.285 inches. Thus, the top surface of the wafer 11 rises slightly above the top surface of the shoulder 4, which helps to maintain laminar gas flow over the wafer. An outer circumference 5 of the grid G is separated from an inner edge 6 of the shoulder 4 by an annular groove 7, which is approximately semicircular in cross section. The depth of annular groove 7 into the susceptor 100 is about the same as the depth of the grid grooves. The diameter of the inner edge 6 of the shoulder 4 is slightly larger than the diameter of the wafer 1 to allow tolerance for positioning the wafer in the pocket. Similar gridded susceptors are commercially available from ASM America, Inc. of Phoenix, Ariz. for use in its Epsilon™ series of CVD reaction chambers.
In FIG. 1A, the wafer 1 is centered over the pocket with equal spacing between wafer edge 2 and shoulder edge 6 all around the wafer. However, as shown in FIG. 1B, upon initial placement, the wafer 1 tends to slide and/or jump, and its outer edge 2 often makes contact with or comes in close proximity to the inner edge 6 of the shoulder 4. The shoulder 4 is thicker and thus generally cooler than the wafer 1 and the underlying grid G. As a result, the edge 2 of the wafer tends to cool by conduction. The wafer edge 2 will also lose heat through radiation if it is very near to the shoulder edge 6, even if they are not actually in contact.
Cooling at the wafer edge renders the temperature of the wafer non-uniform. Given that thin film deposition rates (and many other fabrication processes) are strongly temperature dependent, especially for CVD in the kinetic regime, film thickness, and resistivity, will be non-uniform across a wafer processed under conditions of temperature non-uniformity. Consequently, there is a need for an improved substrate support that facilitates substrate pick-up and drop-off while promoting temperature uniformity.
In satisfaction of this need and in accordance with one aspect of the invention, a substrate support is provided with a grid of grooves extending into a concave surface, which can hold a generally flat substrate, such as a silicon wafer, for processing. The concavity and grooves are configured to minimize stick, slide and curl, while still maintaining desirable thermal properties. Methods for configuring the support and for supporting a substrate thereon are also provided.
In accordance with another aspect of the invention, centering locators, arranged radially along an inside edge of an annular shoulder of a substrate holder, are supplied. The centering locators establish a distance between the substrate and the annular shoulder to prevent direct thermal contact therebetween.